Blood Counts -- this is a good site—explains the counts and also has info on manual and automated counts… Here is a bit from the link above.
Sources of error in manual WBC counting are due largely to variance in the dilution of the sample and the distribution of cells in the chamber, as well as the small number of WBCs that are counted. For electronic WBC counts and differentials, interference may be caused by small fibrin clots, nucleated red blood cells (RBCs), platelet clumping, and unlysed RBCs. Immature WBCs and nucleated RBCs may cause interference with the automated differential count. Automated cell counters may not be acceptable for counting WBCs in other body fluids, especially when the number of WBCs is less than 1000/μL or when other nucleated cell types are present.
For information on Shwachman-Diamond Syndrome check out Shwachman-Diamond America